At the very least middle traps, rhomboids and posterior delts need to be worked because these are usually weak and underutilized. (fitgirlsdiary.com)
In the peak position, as the bar is approaching your torso, your upper arms should be perpendicular to it and parallel to the floor, and you should be able to draw a line from your elbow across your upper lats, rhombs and middle traps. (fitnessandpower.com)
More specifically, behind - the - back shrugs will engage more of your middle traps and help you add thickness, while front shrugs emphasize upper traps. (fitnessandpower.com)